Coating formulation for the interior surfaces of cans

ABSTRACT

The invention relates to a water-based can inner coating comprising a copolymer or a copolymer mixture of at least one aliphatic and acyclic alkene with at least one α,β-unsaturated carboxylic acid in water-dispersed form, wherein the acid number of the copolymer or of the copolymer mixture is at least 20 mg KOH/g, but not more than 200 mg KOH/g, and at least one water-dispersed or water-soluble curing agent. Inventive can inner coatings are characterized in that due to the good crosslinking of the copolymer or of the copolymer mixture with the curing agent, the cured film on the inner surfaces of metal cans possesses excellent properties in regard to hardness, abrasion resistance and resistance towards hot liquids.

This application is a continuation of PCT/EP2012/053830 filed 7 Mar.2012, which claims priority to EP11160695.0 filed 31 Mar. 2011.

The present invention relates to a water-based can inner coatingcomprising a copolymer or a copolymer mixture of at least one aliphaticand acyclic alkene with at least one α,β-unsaturated carboxylic acid inwater-dispersed form, wherein the acid number of the copolymer or of thecopolymer mixture is at least 20 mg KOH/g, but not more than 200 mgKOH/g, and at least one water-dispersed or water-soluble curing agentselected from the group of aminoplasts and/or the group ofcarbodiimides. Inventive can inner coatings are characterized in thatdue to the good crosslinking of the copolymer or of the copolymermixture with the curing agent, the cured film on the inner surfaces ofmetal cans possesses excellent properties in regard to hardness,abrasion resistance and resistance towards aqueous liquids. The presentinvention makes available an alternative to the conventional use ofepoxides based on bisphenols in can inner coatings.

In the food industry, tin plate strip is valued as a suitable materialfor the production of packaging units for receiving aqueous liquids orpreserved foods. This is because, even over a longer period of time, tinplate strip, due to its electrochemically noble tin layer, releases onlylow amounts of potentially toxic tin salts to the food product that isin contact with the tin surface. Tin plate strip is therefore animportant starting material for food packaging, for example for theproduction of cans for receiving beverages. Aluminum strip, due to itspassive oxide layer, is also a suitable starting material for theproduction of cans for filling with beverages. In addition, aluminumsalts that are taken up in small amounts by the liquid are harmless tohealth.

The packaging industry, when producing cans, coats the inner surface ofthe can with an organic protective layer or alternatively uses a stripmaterial pre-coated with an organic protective layer for producing cans.The organic finish that coats the inner surface prevents any directcontact of the metallic interior of the can with the liquid. Thisachieves first of all a significantly reduced corrosion of the basematerial and secondly minimizes the entry of metal salts, such that thetaste of the foodstuff is not changed for the worse even after a lengthystorage or stockpiling of the beverage cans.

Another aspect in regard to the production of cans concerns thecomposition of the coating, which conventionally consists of epoxyresins based on Bisphenol A. Such epoxides with a Bisphenol A basicstructure are suspected estrogens and are teratogens for males. Curedcoating formulations that come into contact with acid-containing aqueousfoodstuffs can release Bisphenol A from the coating into the storedfoodstuff. In practice, the curing of the coating and the resultingcrosslinking of the coating components is also never complete, such thatunreacted Bisphenol A-based epoxides can also diffuse into thefoodstuff. Consequently there exists a need for Bisphenol A-freeformulations for the inner coating of cans for storing foodstuffs;various national legislation initiatives, driven inter alia by the EUDirective 2002/72/EU, exist that define the maximum limits for themigration of Bisphenol A from packaging into foodstuffs.

US 2008/0193689 discloses an epoxide-based coating composition that issuitable for use as a can coating and comprises, in addition to theepoxy resin, mono and difunctional low molecular weight organiccompounds that can react with the epoxy resin. The coating is formulatedin such a way that after curing, only very minor amounts of unreactedepoxides based on Bisphenol A remain in the coating, such that when thecomposition is used as a can inner coating only traces of Bisphenol Afrom the cured coating can migrate into the stored foodstuff.

On the other hand, EP 2031006 proposes can inner coatings based onspecific alicyclic epoxides, so as to circumvent in this way theformulations that include epoxides based on Bisphenol A.

WO 2006/045017 provides a beverage can coating formulation that containslatices of ethylenically unsaturated monomers and an aqueous dispersionof an acid-functional polymer in the presence of amines, wherein thelatices for the crosslinking are constructed at least partially frommonomers having a glycidyl group. Such can inner coatings can beformulated free of epoxides based on Bisphenol A.

The object of the present invention consists in providing anotheralternative to an epoxide-based can inner coating, wherein the coatingformulation can be deposited on the inner surfaces of the can in a sprayprocess and after curing affords thin, homogeneous, highly flexiblecoating films with a simultaneously good coating adhesion and resistancetowards aqueous compositions. Another object consists in being able asfar as possible to obviate the use of organic solvents and emulsifiersin the formulation of stable and coatable can inner coatings.

This object is achieved by a water-based can inner coating comprising,in addition to water,

-   a) a copolymer or a copolymer mixture of at least one aliphatic and    acyclic alkene with at least one α,β-unsaturated carboxylic acid in    water-dispersed form, wherein the acid number of the copolymer or of    the copolymer mixture is at least 20 mg KOH/g, but not more than 200    mg KOH/g, and-   b) at least one water-dispersed or water-soluble curing agent    selected from the group of aminoplasts and/or the group of    carbodiimides.

Cans are inventively understood to mean metallic containers for filling,storing and holding stocks of foodstuffs, in particular of beverages.

In this context, a can inner coating is a coating formulation that forthe formation of a coating layer on the inner surface of the can isdeposited, made into a film and cured in order to prevent the directcontact of the foodstuff with the metallic material of the can duringfilling, storing and holding stocks of the foodstuff.

A water-based coating inventively contains a dispersion and/or emulsionof organic polymers in a continuous aqueous phase, wherein in thecontext of the present invention, an aqueous phase is also understood tomean a homogeneous mixture of water and a water-miscible solvent. Theterm “in water-dispersed form” therefore means that each polymer isdispersed as a solid or liquid in the continuous aqueous phase.

According to the invention, mixtures of chemically and/or structurallydifferent copolymers of at least one aliphatic and acyclic alkene withat least one α,β-unsaturated carboxylic acid constitute a copolymermixture. Thus, a copolymer mixture of an inventive coating formulationcan for example comprise in parallel copolymers that comprise differentalkenes or different α,β-unsaturated carboxylic acids as the comonomersor have a different number of otherwise identical comonomers in thecopolymer.

The acid number is inventively an experimentally measurablecharacteristic number that reflects the number of the free acid groupsin the copolymer or in the copolymer mixture.

The acid number is determined by dissolving a weighed quantity of thecopolymer or the copolymer mixture in a solvent mixture of methanol anddistilled water in the volume ratio 3: 1, and subsequentlypotentiometrically titrating the mixture with 0.05 mol/l KOH inmethanol. The potentiometric measurement is carried out with acombination electrode (LL-Solvotrode® from Metrohm; referenceelectrolyte: 0.4 mol/l tetraethylammonium bromide in ethylene glycol).Here, the acid number corresponds to the added quantity of KOH inmilligrams per gram copolymer or copolymer mixture at the inflectionpoint of the potentiometric titration curve.

As a melted on, thin film on metal surfaces the copolymer or copolymermixture of the aliphatic and acyclic alkane with an α,β-unsaturatedcarboxylic acid with the abovementioned acid number already shows a goodcoating adhesion, in particular on tin plate and aluminum surfaces. Inaddition, the acid groups impart the inherent characteristic to thecopolymer or to the copolymer mixture of being self-emulsifying, suchthat in the aqueous phase, even in the absence of emulsifiers,microparticulate aggregates can be formed by using shear forces. Thepresence of the copolymer or copolymer mixture in the form of amicroparticulate aggregate lends thixotropic properties to the inventivecoating, such that a homogeneous wet film of the water-based coating canbe deposited onto the inner surface of the can, the coating remainingthere until a film is formed and cured, and does not run off inside thecan due to the force of gravity.

If the acid number of the copolymer or copolymer mixture of alkenes andα,β-unsaturated carboxylic acids is less than 20 mg KOH/g, then a curedcoating formulation according to the art of the present invention doesnot have sufficient adhesion to metal surfaces and consequently is notsuitable as a film-forming component of can inner coatings. Conversely,an acid number of the copolymer or copolymer mixture of alkenes andα,β-unsaturated carboxylic acids greater than 200 mg KOH/g as thefilm-forming component in can inner coatings only brings about aninadequate barrier effect against corrosively acting ions in aqueousmedia and furthermore a coating that is comparatively less resistantagainst water at temperatures above 60° C.

The weight fraction of the aliphatic and acyclic alkenes in thecopolymer or in the copolymer mixture is preferably at least 40 wt %,particularly preferably at least 60 wt %, but preferably not more than95 wt %. This ensures that the ion-permeability of the cured coating onthe can inner surface and the swelling of the coating in contact withaqueous media, with at the same time an adequate wettability andadhesion of the coating to the material of the can, is reduced as muchas possible.

Preferred aliphatic and acyclic alkenes of the inventively obtainedcopolymer or copolymer mixture are selected from ethene, propene,1-butene, 2-butene, isobutene, 1,3-butadiene and/or2-methylbuta-1,3-diene, particularly preferably ethene.

Preferred α,β-unsaturated carboxylic acids of the inventively obtainedcopolymer or copolymer mixture are selected from cinnamic acid, crotonicacid, fumaric acid, itaconic acid, maleic acid, acrylic acid and/ormethacrylic acid, particularly preferably acrylic acid and/ormethacrylic acid, in particular acrylic acid.

Further comonomers that may be an additional component of the copolymeror the copolymer mixture in an inventive can inner coating are selectedfrom esters of α,β-unsaturated carboxylic acids, preferably linear orbranched alkyl esters of the acrylic acid and/or methacrylic acidcontaining not more than 12 carbon atoms in the aliphatic group. Suchcomonomers improve the adhesion of the cured inner coating of the can tometal surfaces due to an increased mobility of the polymer backbonewhich again facilitates the orientation of the acid groups that have asurface affinity to the metal surface. This effect is ensured inparticular with low acid numbers of the copolymer below 100 mg KOH/g. Itis generally the case that low acid numbers of the copolymer orcopolymer mixture improve the barrier properties of the cured inventivecoating formulation when exposed to aqueous media. Accordingly,copolymers or copolymer mixtures that additionally comprise the abovedescribed comonomers are inventively preferred with acid numbers below100 mg KOH/g, particularly below 60 mg KOH/g.

The copolymer or the copolymer mixture of the inventive can innercoating preferably comprises less than 0.05 wt %, particularlypreferably less than 0.01 wt of epoxidically bonded oxygen.

A good film-formation when curing the can inner coating requires thatthe water-dispersed copolymer or the water-dispersed copolymer mixtureof the can inner coating is converted into a melted state after theaqueous phase has been driven off. This requirement is satisfied whencopolymers or copolymer mixtures are preferred that as such have a glasstransition temperature of not more than 80° C., particularly preferablynot more than 60° C. Copolymers or copolymer mixtures with a weightaverage molecular weight M_(w) of not more than 20 000 u and which arebased on alkenes and α,β-unsaturated carboxylic acids usually have glasstransition temperatures that are significantly below 100° C., such thatcopolymers or copolymer mixtures with a weight average molecular weightof not more than 20 000 u, in particular not more than 15 000 u, arepreferred in inventive can inner coatings.

In a preferred formulation of the inventive can inner coating, the acidgroups of the water-dispersed copolymer or the water-dispersed copolymermixture are at least partially neutralized. This measure increases theability of the copolymer for self-emulsification in the aqueous phase,such that more stable coating formulations result with lower particlesizes of the dispersed copolymers. Accordingly, the can inner coatingpreferably additionally comprises a neutralizing agent. Preferredsuitable neutralization agents that are additionally comprised in such apreferred formulation are ammonia, amines, metallic aluminum and/orzinc, preferably in powdered form, as well as water-soluble oxides andhydroxides of the elements Li, Na, K, Mg, Ca, Fe(II) and Sn(II). Theperson skilled in the art is aware here that the neutralization agents,corresponding to their function, enter into neutralization reactionswith the components of the inventive coating, and therefore in thesepreferred formulations are optionally detectable as such only indirectlyin the form of their reaction products. For example, metallic aluminumpowder or zinc powder reacts in the aqueous phase, giving off hydrogen,to afford the corresponding hydroxides that again neutralize the acidgroups of the copolymer or copolymer mixture, such that in the inventivecoating finally only the cations of the elements aluminum or zinc can bedetected. The neutralization agents are therefore understood to besolely as formulation aids of the inventive can inner coating. Ammoniaand amines are particularly preferred neutralization agents, as theypass into the gas phase when the coating is cured at elevatedtemperatures and therefore do not remain in the cured can inner coating.Preferred amines that can be employed as the neutralization agent ininventive can inner coatings are morpholine, hydrazine, hydroxylamine,monoethanolamine, diethanolamine, triethanolamine, dimethylethanolamineand/or diethylethanolamine.

The degree of neutralization of the acid groups in the copolymer orcopolymer mixture in the inventive can inner coating is such that atleast 20%, particularly preferably at least 30% of the acid groups areneutralized. High degrees of neutralization above 70%, preferably above60%, are to be avoided in a preferred embodiment of the can innercoating, as the almost completely neutralized copolymers are alreadydissolved in significant amounts in water, thereby resulting again in ahigh viscosity of the coating and average particle sizes of thedispersed copolymer or copolymer mixture in the sub-micrometer range,such that these kinds of formulations are less suitable as the can innercoating due to their rheological properties.

In this context, the neutralization agent to the can inner coating ispreferably to be formulated in such an amount that, based on 1 g ofcopolymer or copolymer mixture, at least 4/z μmol, preferably at least6/z μmol, each multiplied by the acid number of the copolymer orcopolymer mixture, are comprised as the neutralization agent, butpreferably not more than 12/z μmol, particularly not more than 10/zμmol, multiplied by the acid number of the copolymer or copolymermixture. The divisor z is a natural number and corresponds to theequivalent number of the neutralization reaction. The equivalent numberrepresents how many moles of acid groups of the copolymer or copolymermixture are neutralized by one mole of the neutralization agent.

The inventive can inner coating comprises a water-dispersed orwater-soluble curing agent from the group of aminoplasts and/or thegroup of carbodiimides. The curing agent enables the copolymer or thecopolymer mixture to crosslink in a condensation reaction and thus toform a cured coating film on the inner surface of the can. The barrierproperties of the cured inventive can inner coating as a film arecomparable with those of cured epoxide-based coating films.

In the inventive coating the curing agent must have the characteristicthat it crosslinks the copolymer or copolymer mixture through acondensation reaction only at temperatures above the glass transitiontemperature, preferably only above 100° C., as otherwise, curing wouldalready occur before the dispersed polymeric components of the coatingcould form a complete film on the inner surface of the can, thusproducing very heterogeneous coating films.

Particularly suitable aminoplast curing agents are based on melamine,urea, dicyandiamide, guanamines and/or guanidine. In inventive can innercoatings, the aminoplast curing agents are particularly preferablymelamine-formaldehyde resins with a molar ratio of formaldehyde:melamine that is preferably greater than 1.5.

Alternatively or in addition, the curing agent of the inventive caninner coating is a carbodiimide. According to the invention,carbodiimides possess at least one diimide structural moiety of the—C═N═C— type. However, they are preferably polyfunctional with a diimideequivalent weight in the range of 300-500 grams of the polyfunctionalcompound per mole of diimide groups. Particularly preferredcarbodiimides result from isocyanates with at least two isocyanategroups by decarboxylation, in particular those of the general Formula(I):

with n: a whole natural number between 1 and 20;

-   -   R₁ an aromatic, aliphatic or alicyclic residue with not more        than 16 carbon atoms.

The isocyanate groups are additionally preferably blocked withhydrophilic protective groups that as such lend an improvedwater-dispersibility or water-solubility to the carbodiimide. The use ofthese preferred carbodiimides furnishes the additional advantage thatthe can inner coating can be formulated to be almost free of organicsolvents as these carbodiimides are highly water-soluble without alreadycrosslinking the copolymer or copolymer mixture in the aqueousformulation. In a preferred embodiment of an inventive can inner coatingthat at least partially comprises carbodiimides as the curing agent, thecontent of organic solvents is therefore less than 10 wt %, particularlypreferably less than 4 wt %, in particular the can inner coatingpreferably comprises no solvent. Exemplary suitable protective groupswith hydrophilic character are hydroxyalkyl sulfonic acids, hydroxyalkylphosphonic acids, hydroxyalkyl phosphoric acids, polyalkylene glycols aswell as quaternary aminoalkyl alcohols and aminoalkylamines. In aparticularly preferred embodiment of the can inner coating, the curingagent is therefore selected from carbodiimides with blocked terminalisocyanate groups according to the general structural Formula (II):

with n: a whole natural number between 1 and 20;

-   -   R₁: an aromatic, aliphatic or alicyclic residue with not more        than 16 carbon atoms.    -   X: —NH—R₁—N(R₁)₂, —O—R₁—N(R₁)₂, —NH—R₁—N(R₁)₃Y, —O—R₁—N(R₁)₃Y,        —O—R₁—SO₃Z, —O—R₁—O—PO₃Z, —O—R₁—PO₃Z, —O—(C₂H₄)_(p)—OH,        —O—(C₃H₆)_(p)—OH        -   with Y: hydroxide, chloride, nitrate, sulfate        -   with Z: hydrogen, ammonium, alkali metal or alkaline earth            metal        -   with p: a whole natural number between 1 and 6

Preferred diisocyanates that are afforded by decarboxylation of thecorresponding carbodiimides are for example hexamethylene diisocyanate,cyclohexane-1,4-diisocyanate, xylylene diisocyanate, isophoronediisocyanate, dicyclohexylmethane-4,4-diisocyanate, methylcyclohexanediisocyanate and tetramethylxylylene diisocyanate, 1,5-naphthylenediisocyanate, 4,4-diphenylmethane diisocyanate,4,4-diphenyldimethylmethane diisocyanate, 1,3-phenylene diisocyanate,1,4-phenylene diisocyanate, 2,4-toluenylene diisocyanate,2,6-toluenyfene diisocyanate.

Basically, the weight average molecular weight M_(w) of the curing agentin the inventive can inner coating is preferably not more than 2500 u,particularly preferably not more than 1500 u, in order to ensure anadequate crosslinking with the copolymer or the copolymer mixture.

The flow properties of the inventive can inner coating are preferably tobe adjusted, such that on the one hand to enable the coating to beapplied in a spray process and especially in the airless process (whichillustrates an airless atomization spray process) that is usually usedin the beverage can industry. On the other hand, the wet film depositedon the inner surface of the can must not immediately run off due togravitational forces, thereby causing an inhomogeneous coating. Optimumflow properties with good film formation of the dispersed constituentsare obtained for inventive can coatings, whose dispersed polymericconstituents of the water-based coating preferably have a D₉₀ value ofnot more than 100 μm, particularly preferably not more than 60 μm,wherein the D₅₀ value is preferably not less than 1 μm, particularlypreferably not less than 10 μm. The D₉₀-value, respectively theD₅₀-value, means that 90 vol % respectively 50 vol % of the dispersedparticles of the can inner coating are smaller than the specified value.

The D₉₀-value, respectively the D₅₀-value, can be determined from volumeweighted cumulative particle size distributions, wherein the particlesize distribution curve can be measured with the help of lightscattering methods.

The viscosity of the can inner coating is preferably such that a flowtime between 20 and 40 seconds results, when measured with a 4 mm DINflow cup of DIN EN ISO 2431. If the viscosity, measured as the flow timefrom the normalized flow cup, is in this range, then the coating,present as a thin film on the inside of the can, has a flow behaviorthat reduces any run off of the wet film and simultaneously ensures thatthe can inner coating is able to be applied in spray processes.

Emulsifiers that support the dispersion of the copolymer or thecopolymer mixture can be added as an auxiliary to the inventive caninner coating. At least 0.1 wt % of emulsifiers are preferably added forthis purpose. Preferably, non-ionic amphiphiles with an HLB value of atleast 8 can be additionally comprised as the emulsifiers in the caninner coating.

According to the present invention, the HLB value is calculated by thefollowing formula and can assume values of zero to 20 on an arbitraryscale:

-   HLB=20 (M_(l)/M)-   with M_(l) molecular weight of the lipophilic group of the    amphiphile    -   M: molecular weight of the amphiphile

The content of this added auxiliary emulsifier in the can inner coatingis preferably not more than 5 wt %, particularly preferably not morethan 2 wt %. However, the copolymer or the copolymer mixture used in theinventive can inner coating is characterized in that it alreadypossesses self-emulsifying properties as a result of its acid groups.Moreover, it has been shown that the addition of emulsifiers frequentlycauses a decreased adhesion of the cured can inner coating to the tinplate and aluminum surfaces. Accordingly, in a preferred embodiment ofthe can inner coating, in the case that the acid number of the copolymeror copolymer mixture is greater than 60 mg KOH/g, preferably greaterthan 80 mg KOH/g, or the degree of neutralization of the copolymer orcopolymer mixture with an acid number below 100 mg KOH/g is at least30%, then less than 0.1 wt %, particularly preferably less than 0.01 wt% and especially preferably no emulsifiers are comprised that are basedon the non-ionic amphiphiles with an HLB value of at least 8.

Alternatively or in addition to the added emulsifiers, the inventive caninner coating may comprise water-miscible organic solvents that decreasethe polarity of the aqueous phase, so as to induce the emulsification ofthe copolymer or copolymer mixture. For this purpose, at least 1 wt % ofwater-miscible organic solvents are added. In this regard, the boilingpoint of the water-miscible solvent under standard conditions ispreferably not more than 150° C.

Suitable solvents are glycol ethers, alcohols and esters. The content ofthe solvent in the can inner coating is preferably not more than 40 wt%, particularly preferably not more than 20 wt %.

Inventive can inner coatings may comprise wetting agents, levelingagents, defoamers, catalysts, film-formers, stabilizers and/or thealready mentioned neutralizing agents as additional constituents. Thesekinds of auxiliaries are generally known to the person skilled in theart of coating objects, wherein film-formers in the present inventionare understood to mean organic polymers that can crosslink with thecuring agent present in the can inner coating. The content by weight offilm-formers based on the copolymer or copolymer mixture is at most 20%,preferably at most 10%.

A preferred formulation of an inventive can inner coating comprises, inaddition to at least 40 wt % water,

-   -   a) 4-30 wt %, preferably 10-20 wt %, of the copolymer or of the        copolymer mixture in dispersed form,    -   b) 2-20 wt. %, preferably 4-12 wt. %, of the at least one curing        agent,    -   c) not more than 5 wt % of emulsifiers selected from non-ionic        amphiphiles with an HLB value of at least 8;    -   d) not more than 40 wt %, preferably at least 1 wt %, of        water-miscible organic solvents;    -   e) not more than 10 wt. % of auxiliaries selected from wetting        agents, leveling agents, defoamers, catalysts, film-formers,        stabilizers and/or neutralizing agents, preferably not more than        12/z μmol of neutralization agent multiplied by the acid number        of the copolymer or copolymer mixture are comprised per gram of        the copolymer or copolymer mixture where z is the equivalent        number of the relevant neutralization reaction.

A particularly preferred reduced-solvent formulation of an inventive caninner coating comprises, in addition to at least 40 wt % water,

-   -   a) 4-30 wt %, preferably 10-20 wt %, of the copolymer or of the        copolymer mixture in dispersed form,    -   b) 2-20 wt %, preferably 4-12 wt % of at least one resin, of        which at least 40 wt % of a carbodiimide with terminal, blocked        isocyanate groups based on the total content of the curing        agent,    -   c) not more than 5 wt % of emulsifiers selected from non-ionic        amphiphiles with an HLB value of at least 8;    -   d) not more than 10 wt %, preferably not more than 1 wt %, of        water-miscible organic solvents;    -   e) not more than 10 wt % of auxiliaries selected from wetting        agents, leveling agents, defoamers, catalysts, film-formers,        stabilizers and/or neutralizing agents, preferably not more than        12/z μmol of neutralization agent multiplied by the acid number        of the copolymer or copolymer mixture are comprised per gram of        the copolymer or copolymer mixture where z is the equivalent        number of the relevant neutralization reaction.

Inventive can inner coatings are characterized in that due to the goodcrosslinking of the copolymer or of the copolymer mixture with thecuring agent, the cured film on the inner surfaces of metal canspossesses excellent barrier properties. The metallic base material isconsequently firstly effectively protected against corrosion andsecondly the liquid stored in the can will not take up any extraneoussubstance. Therefore, the present invention makes available analternative to the conventional use of epoxides in can inner coatings,in particular epoxides based on Bisphenol A. Consequently, the contentof epoxidically bonded oxygen in inventive can inner coatings ispreferably not more than 0.1 wt %, particularly preferably not more than0.01 wt %. An inventive can inner coating particularly preferablycomprises no organic constituents with epoxide groups.

Inventive can inner coatings can preferably be produced in closedprocesses in pressure reactors using shear forces, wherein allconstituents of an inventive can inner coating are transferred into apressure reactor, in order to be subsequently subjected to a shear rateof at least 1000 s⁻¹ at temperatures in the range of 80-200° C. and apressure of 1-6 bar, wherein the energy input is preferably in the rangeof 10³-10⁵ J per second per liter of coating formulation. Alternatively,the solid constituents together with the usual components of the caninner coating are also dispersed in an open process, in which the meltedcopolymer or the melted copolymer mixture under the action of theabovementioned shear force is transferred into the aqueous compositionof the usual can inner coating composition. However, the shear rate andresidence time in each dispersion process is preferably adjusted suchthat the dispersed constituents of the can inner coating have a D₉₀value of not more than 100 μm, wherein the D₅₀ value is preferably notbelow 1 μm, particularly preferably not below 10 μm.

The application of a wet film of the inventive can inner coating ispreferably carried out in a spray process, particularly preferably inthe “Airless Process”, in which the can inner coating is airlesslyatomized and thus deposited onto the material surface. In this sprayprocess, a defined quantity of the can inner coating is introduced intothe cleaned and dried can interior by means of spray guns, while the canis rotated about its own Longitudinal axis in order to form ahomogeneous film. The wet film on the can inner surface is then cured toa coating film in a drying oven at temperatures ranging between 120° C.and 200° C. (object temperature). The curing process includes thevolatilization of the aqueous phase as well as the film formation andcrosslinking of the polymeric constituents.

In another aspect, the present invention relates to the use of acopolymer or a copolymer mixture of at least one aliphatic and acyclicalkene with at least one α,β-unsaturated carboxylic acid inwater-dispersed form, wherein the acid number of the copolymer or of thecopolymer mixture is at least 20 mg KOH/g, but not more than 200 mgKOH/g, and the acid groups of the copolymer or of the copolymer mixturein the water-dispersed form are at least 20%, but not more than 70%neutralized, as a constituent of water-based can inner coatings, whereinpreferred uses can be realized by above described correspondingembodiments of the copolymer or of the copolymer mixture.

In another aspect, the present invention relates to the use of an abovedescribed can inner coating that is deposited in a dry film thickness ofat least 5 g/m², but preferably of not more than 50 g/m², on to theinner surface of a tin plate can and in a dry film thickness of at least1.5 g/m², but preferably not more than 50 g/m², on to the inner surfaceof an aluminum can.

EXAMPLES

Table 1 lists the compositions of the inventive can inner coatings thatwere deposited as a wet film on to the inner surfaces of tin plate cansby means of spray processes, and then cured for 40 seconds at 180° C. toa dry coating with a coating weight of 6-7 g/m².

The water-based can inner coatings were manufactured in an open reactorby continuously metering the melted copolymer to an aqueous compositionof the remaining constituents under a shear stress of 1500 s⁻¹ at 95° C.After metering in the copolymer, the homogenization was continued in theopen reactor until a constant viscosity of the coating formulation wasachieved. The viscosity of the coating formulations, measured as theflow time from a DIN 4 mm flow cup according to DIN EN ISO 2431 lay inthe range 25-28 seconds.

The coating formulations homogenized in this way were then depositedonto the inner surfaces of the tin plate can in a two-step sprayprocess, wherein the tin plate can was rotated about an axis andinitially the bottom of the can and lower part of the body was coatedand then the can body and end were sprayed. The wet film was then cured.

From Table 2 it can be seen that the tin plate cans coated with theinventive coating possess an excellent flexibility (T-bend test) andwater resistance (Koch test). Solely the hardness and solvent resistancetests showed differing results, which, however, all met the requirementsof the beverage can industry.

TABLE 1 Exemplary formulations of inventive can inner coatingsConstituents in wt % (rest is water) Constituent Compound B1 B2 B3Copolymer Ethylene-acrylic acid; acid no. 37-44 mg KOH/g; Neutralizationdegree 17.1 50% (dimethylethanolamine) Ethylene-acrylic acid; acid no.37-44 mg KOH/g; Neutralization degree — 21.3 21.5 30% (ammonia) Curingagent Melamine/formaldehyde resin partially methylated of the imino type9.0 — 4.5 Polycarbodiimide with diimide eq. wt of 445 g/mol — 4.0 4.0Solvent Monopropylene glycol monomethyl ether 11.0 — 6.0 Butyl glycol9.0 — 1.7 Defoamer Polyether siloxane copolymer 0.63 0.5 0.5 Wettingagent Bis(2-ethylhexyl)sulfosuccinate, Na salt — 0.75 0.75

TABLE 2 Properties of the cured can inner coatings of Table 1 in tinplate cans Property Test B1 B2 B3 Coating adhesion¹ T-bend acc. DIN ISO17132 0 0 0 Cross-hatch test acc. DIN 0 0 0 53151 (24 h) Boil test acc.DIN 53151 0 0 0 (30 min at 85° C.) Coating hardness Pencil hardness acc.DIN HB B B ISO 15184 Solvent resistance MEK Test acc. DIN EN 90  90 100  13523-11 ¹Classification according to coating dissolution in %based on the tested surface 0: no dissolution; 1: 5%; 2: 15%; 3: 35%; 4:<65%; 5: >65%

1. A water-based can inner coating comprising, in addition to water, a)a copolymer or a copolymer mixture of at least one aliphatic and acyclicalkene with at least one α,β-unsaturated carboxylic acid inwater-dispersed form, wherein the acid number of the copolymer or of thecopolymer mixture is at least 20 mg KOH/g, but not more than 200 mgKOH/g, and b) at least one water-dispersed or water-soluble curing agentselected from an aminoplast, a carbodiimide and combinations thereof. 2.The water-based can inner coating according to claim 1, wherein the acidgroups of the copolymer or of the copolymer mixture in water-dispersedform are at least partially neutralized.
 3. The water-based can innercoating according to claim 2, comprising a neutralization agent forneutralizing the acid groups of the copolymer or of the copolymermixture in water-dispersed form, said neutralization agent beingselected from the group consisting of ammonia, amines, metallic Al,metallic Zn, water-soluble oxides of Li, Na, K, Mg, Ca, Fe(II), Sn(II),water-soluble hydroxides of Li, Na, K, Mg, Ca, Fe(II), Sn(II) andcombinations thereof.
 4. The water-based can inner coating according toclaim 3, wherein the neutralization agent is selected from ammonia,morpholine, hydrazine, hydroxylamine, monoethanolamine, diethanolamine,triethanolamine, dimethylethanolamine, diethylethanolamine, andcombinations thereof.
 5. The water-based can inner coating according toclaim 1, wherein the copolymer or the copolymer mixture has a glasstransition temperature of not more than 80° C.
 6. The water-based caninner coating according to claim 1, wherein the aliphatic and acyclicalkene is selected from ethene, propene, 1-butene, 2-butene, isobutene,1,3-butadiene, 2-methylbuta-1,3-diene and combinations thereof.
 7. Thewater-based can inner coating according to claim 1, wherein theα,β-unsaturated carboxylic acids are selected from cinnamic acid,crotonic acid, fumaric acid, itaconic acid, maleic acid, acrylic acid,methacrylic acid and combinations thereof.
 8. The water-based can innercoating according to claim 1, wherein a weight fraction of the aliphaticand acyclic alkenes in the copolymer or in the copolymer mixture is atleast 40 wt. %, but not more than 95 wt. %.
 9. The water-based can innercoating according to claim 1, wherein the copolymer or the copolymermixture additionally comprises comonomers that are selected from estersof α,β-unsaturated carboxylic acids.
 10. The water-based can innercoating according to claim 9, wherein the esters of α,β-unsaturatedcarboxylic acids comprise linear or branched alkyl esters of acrylicacid and/or methacrylic acid, said esters having an aliphatic group ofnot more than 12 carbon atoms, wherein the copolymer or the copolymermixture has an acid number of less than 100 mg KOH/g.
 11. Thewater-based can inner coating according to claim 1, wherein waterdispersed polymeric constituents of the water-based can inner coatinghave a D90 value of not more than 100 μm and a D50 value of not lessthan 1 μm.
 12. The water-based can inner coating according to claim 1,comprising at least 40 wt. % water and a) 4-30 wt. % of the copolymer orof the copolymer mixture, b) 2-20 wt. % of the at least one curingagent, c) not more than 5 wt. % of emulsifiers selected from non-ionicamphiphiles with an HLB value of at least 8; d) not more than 40 wt. %of water-miscible organic solvents; e) not more than 10 wt. % ofauxiliaries selected from wetting agents, leveling agents, defoamers,catalysts, film-formers, stabilizers and neutralizing agents.
 13. Amethod of coating inner surfaces of a can comprising applying thewater-based can inner coating according to claim 1 to inner surfaces ofa can and drying said water-based can inner coating.
 14. The method ofclaim 13, wherein the can is a tin plate can and the inner coating isdeposited in a dry film thickness of at least 5 g/m², but not more than50 g/m², on to the inner surface.
 15. The method of claim 13, whereinthe can is an aluminum can and the inner coating is deposited in a dryfilm thickness of at least 1.5 g/m², but not more than 50 g/m², on tothe inner surface.
 16. The method of claim 13, wherein the water-basedcan inner coating is deposited in a spraying process.